Need some advice

Tanner is a 4.5 month old sheltie pup...He's doing excellent with basic training, I try to be alpha...I am having a problem in one area that Tanner is challenging me on...When I give him the "down" command with hand signal, he grabs my hand...If I do a distance drop he runs and bites my hand then drops...

If he is off leash and I tell him down, he takes off running in a very playful way and then comes back to me and if I repeat the command he takes off running again...Once he wears himself out he drops when told...Today he bit me not once but 3 times, he got on his run and ran up to me, bit me and took off...this happened 2 more times...

He came back to me and laid down...So it's a combo of him challenging me AND he thinks I'm playing (only with the down tho) but he bit me HARD...He didn't break the skin but it was still hard...Any ideas?...Thanks S14me

At 4.5 months I think you may be expecting a little too much from him with distance drops and so on. You should still be working on bite inhabition, and basic commands. Make sure he has reliable recall (I'm still working on this at 8 months!) down, sit, stay. Or a few basic tricks.
When the bite inhabition is worked out, you won't have to worry about him running up and chomping when you're working on more complicated commands.

Never mind I figured it out on my own lol...S14me